On the football field, Pat Tillman was known for his fierce tackles and unbridled spirit.
"Pat, as a football player, loved the roar of the crowd," best-selling author Jon Krakauer told Bob Woodruff in an exclusive interview earlier this month with ABC News. "He was a gregarious guy."
Off the gridiron, the Arizona Cardinals safety was a different man -- an unflinching patriot who died for his country in Afghanistan.
"It was sort of like there was the public Pat and the private Pat," Krakauer said. "And the private Pat was not for public consumption."
Tillman later symbolized all those who have sacrificed their lives in the fight against terrorism.
Krakauer pieces together Tillman's story in his new book, "Where Men Win Glory: The Odyssey of Pat Tillman," to be released Tuesday.
"Who does this?" Tillman wrote in a journal entry dated July 28, 2002. "Who takes a perfectly perfect life and ruins it? A perfectly happy wife and marriage and jeopardizes it? Ahhh! If I do not strangle someone while I'm here, I was touched by an angel."
Tillman's personal journals and letters to his wife, Marie, seen publicly for the first time, offer a glimpse into the life of the man that very few people knew.
"My life at this point is relatively easy," Pat Tillman wrote in a letter to his family dated April 8, 2002. "It is my belief that I could continue to play football for the next seven or eight years and create a very comfortable lifestyle. My job is challenging, enjoyable and strokes my vanity enough to fool me into thinking it's important. For more reasons that I care to list, my job is remarkable. However, it is not enough."
Tillman's personal journals and letters reveal an intelligent, sensitive and loyal man who loved his wife, family and country.
Like many other Americans, Krakauer admired Tillman's decision to give up his multi-million dollar football career to volunteer for military duty after the Sept. 11 terrorist attacks.
Once Krakauer started working on the book, he realized there was more to Tillman than football hero.
"Only then did I realize his depth and his complexity, and he was a really interesting guy," said Krakauer, who never met Tillman. "He was much more than a football player or a patriot or any of that. He was all those things. But he was a remarkable human being."
Driven by the events of 9/11, Tillman enlisted in the army. In a letter to his family members, Tillman informed them of his decision.
"For much of my life I've tried to follow a path I believed important," he wrote. "Sports embodied many of the qualities I deem meaningful: courage, toughness, strength. These last few years, and especially after recent events, I've come to appreciate just how shallow and insignificant my role is. I'm no longer satisfied with the path I've been following ... it's no longer important."
Not everyone in his family agreed with his decision. Tillman's family organized an intervention.
"This intervention just sort of degenerated into this kind of yelling match and people crying and it was a kind of a disaster," said Krakauer, who wrote the acclaimed "Into Thin Air: A Personal Account of the Mt. Everest Disaster" 10 years ago.
Once her husband's mind was made up, Marie Tillman said, there was no talking him out of it. But Tillman's doubts about his decision surfaced during boot camp, according to Tillman's journal entries.